The present invention relates to an output stage, the type of which is automatically selected depending on the supply voltage. Either a bipolar junction transistor or a field effect transistor is selected, depending upon the supply voltage level. The invention is particularly, though not exclusively, useful for implementing a regulator for an alternator of an electric system of self-generation and recharge of a storage battery.
An output stage, comprising a power transistor capable of driving a load, typically an external load that is connected to a first supply node, by functionally connecting it to a second supply node, represents the output circuit for implementing a certain function (regulation, control, actuation, charge, display, . . . ) in a vast number of applications.
Commonly, the stage may be configured as a so-called "low-side driver", usually employing a bipolar NPN transistor or an n-channel MOS transistor, or as a so-called "high-side driver", usually employing a PNP bipolar transistor or a p-channel MOS transistor.
Stringent requirements to limit power dissipation through the power transistor that drives the external load determine ever more often the choice of a field effect transistor (typically a MOS transistor), which is intrinsically capable of reducing power dissipation by about an order of magnitude as compared with a bipolar junction transistor of similar current-carrying capacity. On the other hand, a power MOS transistor hardly has a ram-on, threshold voltage below about 0.8-1 V and, typically, in the case of a transistor designed for relatively high power, the threshold voltage may raise to a value comprised between 3 V and 6 V.
However, there are many applications, especially in electric self-generation systems, wherein, during certain operating phases, for example at the start-up of the system, the supply voltage may be subject to a large drop. This may occur for example because of momentary large current absorption peaks of certain loads, for example an electric starter of a thermal engine, and similar. In other situations, as for example in the case of an electric equipment of a vehicle, certain specific requisites, such as the need of regulating the voltage powering the electric circuit of the vehicle, as produced by the alternator, already at a very low number of RPM of the thermal engine driving the alternator, may not be compatible with the relatively high threshold voltage of an output power MOS transistor. For example, during a "cold" start-up of a car engine, the voltage provided by the car battery, may momentarily drop by about 5-6 V or more. The use of a power MOS having a relatively high threshold voltage, may cause an interruption of the current delivered to the solenoids of relays that are functional to the operation of the engine or performing other important duties and undue deactivations.
In general, where the requirement of reducing power consumption is combined with the need to prevent unwanted effects of abrupt changes of the supply voltage, a "compatibility" problem may arise. For example, when MOS transistors are used as output power driving elements for reducing power consumption, a reliability problem may arise at cold start-ups.
A typical example of a system of electric self-generation is represented by the electric installation of a vehicle. Normally an alternator, driven by the thermal engine of the vehicle, is employed. Generally, the alternator is a multiphase machine (commonly a three phase machine), with stator windings connected in a star or triangle configuration. The inductor is commonly a rotor winding (field winding). Normally, a regulator is employed for controlling the voltage generated by the alternator.
A typical installation of this type is depicted in FIGS. 3 and 4.
The regulator, through a voltage regulating network formed by R1, R2, DZ1 and Q1, controls the voltage present across its terminals D- and D+ by driving, through a transistor Q2, a current through the field winding (rotor winding) of the alternator, in order to maintain the voltage generated by the alternator as constant as possible, independently of the electric load and of the speed of rotation of the rotor (and of the car engine).
In such a "monofunction" regulator system, an indicator lamp is connected between the cathodes of an auxiliary rectifier bridge and the cathodes of a main rectifier bridge. When the alternator outputs current to the loads of the electric circuit of the car, there isn't any difference of potential between said two nodes and the indicator lamp stays off. When the alternator is not delivering any current, the indicator lamp is lit, through the electric path: battery / starter-key switch / field winding / Q2.
In systems of this type, at start-up, that is when (with the engine and alternator still at rest) the starter-key switch is closed, it is desirable that the indicator lamp should light up, to signal absence of alternator current, and also to circulate a certain current through the field winding (rotor winding) of the alternator, in order to produce a certain pre-excitation of the magnetic circuit of the alternator. A pre-excitation does in fact favor the start-up of the system at a lower number of RPM than by exploiting solely the residual magnetism of the magnetic circuit.
The need for a pre-excitation phase is becoming ever more important in the car industry, where it is highly desirable to be able to regulate the voltage on the electric circuit of the vehicle already at a minimum number of RPM of the thermal engine.
At start-up, the equivalent diagram of the regulation circuit becomes that of FIGS. 5 and 6. Taking into account typical values of impedance of the components of the electric circuit of a car, it may be observed that the voltage at the supply pin (D+) of the regulator may drop as low as to about 1 V, as shown schematically in FIG. 6.
The output stage of a regulator for an alternator, that is the transistor (low-side driver or high-side driver), that controls the current through a field winding of the alternator, is normally constituted by a bipolar junction transistor (BJT) or by a field effect transistor (MOS).
A bipolar power transistor (for example Q2 in the figures) would be capable of ensuring a pre-excitation of the field winding (rotor winding) at start-up, with a supply voltage of about 1 V, because the transistor requires a voltage given by the sum of the voltage drop through the resistance R4 (FIGS. 4, 5 and 6) and of the base-emitter voltage (VBE) of the transistor itself (typically about 0.7 V) as the minimum turn-on voltage.
By contrast, the use of a bipolar transistor has the disadvantage of producing a relatively high power dissipation. In fact, by considering as an example the following typical values: EQU I.sub.field =5A, V.sub.cesat-Q2 =0.25 V, gain.sub.Q2 =10, V.sub.reg =14.4 V,
it may be calculated that the transistor will require a driving current I.sub.driv of about 0.50 A. Therefore, the power dissipation will be given by: EQU P.sub.diss=I *V.sub.cesat +I.sub.driv *V.sub.reg =5*0.25+0.5*14.4=8.45 W.
By contrast, the use of a power MOS in place of a bipolar junction transistor, permits to practically eliminate power dissipation in driving the output transistor (voltage driving). In fact, if for example a power MOS having a series resistance comparable to that of a bipolar transistor, that is: R.sub.dson =0.25 V/5A=0.05.OMEGA., is used, power dissipation will be given by: EQU P.sub.diss =R.sub.dson *I.sub.field *I.sub.field =0.05*5*5=1.25 W.
A low power dissipation is increasingly often required in the automotive industry because a smaller power dissipation means lower junction temperature of the transistors and therefore a higher reliability of the system.
On the other hand, the use of a field effect power transistor (MOS) is not compatible with the other requirement of allowing a smooth start-up also in presence of relatively low values of the supply voltage (D+P.apprxeq.1 V), because, at present, field effect power transistors that can be realized in so-called mixed-technology integrated circuit fabrication processes as well as "discrete" devices for this type of applications, have a threshold voltage of several volts, typically from 3 to 6 V.
There is a need for an output power stage (low-side driver or high-side driver) which not only has a low power dissipation, but also ensures a trouble-free operation of the system even in the presence of large drops in the supply voltage.
A main object of the present invention is to provide an output power stage having a low power dissipation under steady state working conditions and a low turn-on threshold, in order to ensure operability of the system also in presence of large drops of the supply voltage.
A further object of the present invention is to provide an automotive voltage regulator that couples a low power dissipation with the ability to function at relatively low RPM (i.e. low speed of the thermal engine driving the alternator).
These objects and advantages are obtained by an output stage characterized by employing alternatively a bipolar junction power transistor and a field effect power transistor, depending on the actual level of the supply voltage. A drive signal, produced by a voltage regulating network is automatically fed to a control terminal (base) of an output bipolar power transistor or to a control terminal (gate) of an output power field effect Transistor, by a switch driven by a comparator which compares the voltage present on the supply node with a reference voltage.
According to an another aspect of the present invention, a regulation circuit for an alternator of a self-generation and storage battery recharging system of an electric equipment of a vehicle, alternatively employs a junction bipolar transistor having a size designed to control a pre-excitation current during a pre-start-up phase of the system and a field effect power transistor, having a size suitable to control a field excitation current during normal running of the alternator. A drive signal produced by a voltage regulating network is automatically fed to a control terminal (base) of the output bipolar power transistor or to a control terminal (gain) of the output power field effect transistor, by a switch driven by a comparator which compares the voltage present on the supply node with a reference voltage.
The regulator permits a smooth start-up also at low supply voltage, by virtue of the low threshold voltage characteristic of the bipolar transistor, while, once the engine has been started, the consequent rising of the voltage on the supply node eventually causes a change of state of the comparator which switches the driving signal from the base of the bipolar transistor to the gate of the MOS transistor, which is capable thereafter of ensuring operation of the regulator with a low power dissipation.
On the other hand, because the bipolar transistor is destined to handle a relatively low current (pre-excitation current), definitely lower than the current (field winding regulating current) that is handled by the output stage during normal running of the system, the additional silicon area that is eventually required for implementing such an "auxiliary" or parallel output stage is relatively small.
According to a disclosed class of innovative embodiments, there is provided: A power driver circuit comprising: control circuitry connected to provide a logic output in a first state whenever current should be passed between a load connection and a first power supply connection; first and second power transistors each having first and second current-carrying terminals thereof connected between said load connection and said power supply connection; said first transistor having a threshold voltage greater than one Volt, and said second transistor having a threshold voltage less than one Volt; and a comparator and switching circuit connected to monitor a power supply voltage, and accordingly to route said output of said control circuitry to a control terminal of said first transistor whenever the power supply voltage is within acceptable limits, and to a control terminal of said second transistor whenever the power supply voltage is NOT within acceptable limits.
According to another disclosed class of innovative embodiments, there is provided: A voltage regulator circuit, having voltage input terminals and a current output terminal, and comprising: control circuitry connected to monitor the voltage across two voltage input terminals, and accordingly to provide a logic output in a first state whenever the voltage across said voltage input terminals falls below a predetermined threshold value; first and second power transistors each having first and second current-carrying terminals thereof connected between a current output terminal of the regulator and a fixed potential; said first transistor having a threshold voltage greater than one Volt, and said second transistor having a threshold voltage less than one Volt; and a comparator and switching circuit connected to monitor the voltage difference between said input terminals, and accordingly to route said output of said control circuitry to a control terminal of said first transistor whenever said voltage difference is within acceptable limits, and to a control terminal of said second transistor whenever said voltage difference is NOF within acceptable limits.
According to another disclosed class of innovative embodiments, there is provided: An automotive electrical supply system, comprising: an alternator having stator windings and a field winding, said stator windings being operatively connected through clamp diodes to provide a voltage between a system power supply line and a system ground connection; a voltage regulator, having voltage input terminals and a current output terminal connected to drive said field winding of said alternator, comprising: control circuitry connected to monitor the voltage across two voltage input terminals, and accordingly to provide a logic output in a first state whenever the voltage across said voltage input terminals falls below a predetermined threshold value; first and second power transistors each having first and second current-carrying terminals thereof connected between a current output terminal of the regulator and a fixed potential; said first transistor having a threshold voltage greater than one Volt, and said second transistor having a threshold voltage less than one Volt; and a comparator and switching circuit connected to monitor the voltage difference between said input terminals, and accordingly to route said output of said control circuitry to a control terminal of said first transistor whenever said voltage difference is within acceptable limits, and to a control terminal of said second transistor whenever said voltage difference is NOT within acceptable limits; a battery connected to provide voltage between said system power supply line and ground connection when said alternator is not active; and a pre-excitation load operatively connected in series between said clamp diodes and one terminal of said battery.
According to another disclosed class of innovative embodiments, there is provided: An output stage comprising: a power transistor driving a load, which is connected to a first supply node, by connecting said load to a second supply node, and a control circuit of said power transistor, comprising a bipolar power transistor and a field effect power transistor, functionally connected in parallel to each other, each having an independent control terminal; a comparator having a first input connected to said first supply node, a second input to which a reference voltage is applied and an output connected to a control terminal of a switch alternatively feeding a signal produced by said control circuit to the control terminal of said bipolar junction transistor or to the control terminal of said field effect transistor.
According to another disclosed class of innovative embodiments, there is provided: A regulating circuit for the output voltage of an alternator of an electric installation of self-generation and recharge of a storage battery, capable of forcing a pre-excitation current through a field winding of the alternator upon the closing of a startup switch of the system and to regulate the excitation current through said field winding of the running alternator, comprising: a driver transistor having a control terminal connected to the output of a voltage regulating network of the circuit, said field winding and said regulating network being powered, through said start-up switch, by said battery, a bipolar transistor and a field effect transistor, functionally connected in parallel to each other, each having an independent control terminal; a comparator having a first input connected to the supply node of said voltage regulating network, a second input to which a constant reference voltage is applied and an output connected to a control terminal of a switch capable of connecting the output of said voltage regulating network to the control terminal of said bipolar transistor or to the control terminal of said field effect transistor.
According to another disclosed class of innovative embodiments, there is provided: A method for operating an output stage, comprising the steps of: providing a power field-effect transistor in parallel with a power bipolar transistor; monitoring a power-supply voltage level; and selectively driving said bipolar transistor but not said field-effect transistor, if said monitoring step indicates that the magnitude of the power-supply voltage level is above a predefined minimum level, and otherwise selectively driving said field-effect transistor but not said bipolar transistor, if said monitoring step indicates that the magnitude of the power-supply voltage level is below said predefined minimum level.
According to another disclosed class of innovative embodiments, there is provided: A method for operating an automotive voltage regulator, comprising the steps of: generating current in an engine-driven alternator having a field coil and stator coils; monitoring the system power-supply voltage, and accordingly providing drive current to said field coil, when required, through a field-effect transistor if the system power-supply voltage is below a target voltage but not below a low-voltage limit, and providing drive current to said field coil, when required, through a bipolar transistor if the system power-supply voltage is below a target voltage and below said low-voltage limit.